Selective para-xylene production via methylation of toluene with methanol in the presence of modified HZSM-5 catalyst

ABSTRACT

We disclose a method for converting toluene to xylenes, comprising contacting toluene with methanol in the presence of a silica-bound HZSM-5 catalyst. As an example, in one embodiment the method can include: (i) first silylating HZSM-5, to form silylated HZSM-5; (ii) first calcining the silylated HZSM-5, to form calcined silylated HZSM-5; (iii) binding the calcined silylated HZSM-5 to silica, to form silica-bound calcined silylated HZSM-5; (iv) extruding the silica-bound calcined silylated HZSM-5, to form extruded silica-bound calcined silylated HZSM-5; (v) second calcining the extruded silica-bound calcined silylated HZSM-5, to form extruded silica-bound twice-calcined silylated HZSM-5; (vi) second silylating the extruded silica-bound twice-calcined silylated HZSM-5, to form extruded silica-bound twice-calcined twice-silylated HZSM-5; and (vii) third calcining the extruded silica-bound twice-calcined twice-silylated HZSM-5, to form the silica-bound HZSM-5 catalyst.

BACKGROUND OF THE INVENTION

[0001] The present invention relates generally to the field of xylenesproduction. More particularly, it concerns methods for convertingtoluene to xylenes using catalyst systems capable of producing p-xylenein high yield.

[0002] Methylation of toluene with methanol is known in the art as auseful technique for the formation of xylenes. The reaction has thefollowing general formula (Formula I):

[0003] wherein x, y, and z are each between 0 and 1, inclusive, andx+y+z=1.

[0004] From top to bottom on the right of Formula I are ortho-xylene(o-xylene, 1,2-dimethylbenzene), meta-xylene (m-xylene,1,3-dimethylbenzene), and para-xylene (p-xylene, 1,4-dimethylbenzene).Of the three, p-xylene is particularly useful in making, either directlyor by substitution at the methylene moieties, straight-chain polymerssuch as polyethylene terephthalate (PET). Generally, however, themethylation of toluene produces all three of the xylene isomers shown inFormula I. Although the separation of p-xylene from a raw productmixture containing the other xylene isomers is possible, it will beapparent that techniques for maximizing the fraction of the raw productmixture defined by p-xylene (i.e., increasing z in Formula I) arebeneficial.

SUMMARY OF THE INVENTION

[0005] In one embodiment, the present invention relates to a method forconverting toluene to xylenes, comprising contacting toluene withmethanol in the presence of a silica-bound HZSM-5 catalyst.

[0006] In another embodiment, the present invention relates to a methodfor converting toluene to xylenes, comprising contacting toluene withmethanol in the presence of a silica-bound HZSM-5 catalyst, wherein thecatalyst can be prepared by a process comprising: (i) first silylatingHZSM-5, to form silylated HZSM-5; (ii) first calcining the silylatedHZSM-5, to form calcined silylated HZSM-5; (iii) binding the calcinedsilylated HZSM-5 to silica, to form silica-bound calcined silylatedHZSM-5; (iv) extruding the silica-bound calcined silylated HZSM-5, toform extruded silica-bound calcined silylated HZSM-5; (v) secondcalcining the extruded silica-bound calcined silylated HZSM-5, to formextruded silica-bound twice-calcined silylated HZSM-5; (vi) secondsilylating the extruded silica-bound twice-calcined silylated HZSM-5, toform extruded silica-bound twice-calcined twice-silylated HZSM-5; and(vii) third calcining the extruded silica-bound twice-calcinedtwice-silylated HZSM-5, to form the silica-bound HZSM-5 catalyst.

[0007] In a further embodiment, the present invention relates to amethod for converting toluene to xylenes, comprising contacting toluenewith methanol in the presence of a silica-bound HZSM-5 catalyst, whereinthe catalyst can be prepared by a process comprising: (i) binding HZSM-5to silica, to form silica-bound HZSM-5; (ii) silylating the silica-boundHZSM-5, to form silylated silica-bound HZSM-5; (iii) calcining thesilylated silica-bound HZSM-5, to form calcined silylated silica-boundHZSM-5; (iv) first washing the calcined silylated silica-bound HZSM-5,to form washed calcined silylated silica-bound HZSM-5; (v) first dryingthe washed calcined silylated silica-bound HZSM-5, to form driedcalcined silylated silica-bound HZSM-5; (vi) steaming the dried calcinedsilylated silica-bound HZSM-5, to form steamed calcined silylatedsilica-bound HZSM-5; (vii) treating the steamed calcined silylatedsilica-bound HZSM-5 with acid, to form acid-treated calcined silylatedsilica-bound HZSM-5; (viii) second washing the acid-treated calcinedsilylated silica-bound HZSM-5, to form washed acid-treated calcinedsilylated silica-bound HZSM-5; and (ix) second drying the washedacid-treated calcined silylated silica-bound HZSM-5, to form thesilica-bound HZSM-5 catalyst.

[0008] Other aspects and features of the invention will become apparentin light of the detailed description and the claims.

DESCRIPTION OF ILLUSTRATIVE EMBODIMENTS

[0009] In one embodiment, the present invention relates to a method forconverting toluene to xylenes, comprising: (a) contacting toluene withmethanol in the presence of a silica-bound HZSM-5 catalyst. The catalystof the present invention can be prepared by a number of processes, threeof which will be described below.

[0010] Techniques for contacting toluene with methanol in the presenceof a solid catalyst for the production of xylenes are well known in theart. (The term “xylenes,” when used without an isomer-designatingprefix, refers herein to a mixture of ortho-, meta-, and para-xylenes).Any of a number of well known methods can be used for producing xylenesin light of the present disclosure, and the invention is not limited bythe particular method and reaction parameters used.

[0011] HZSM-5 is known in the art. ZSM-5 is an aluminosilicate zeolitewith a high silica content and a low aluminum content. Its structure canbe considered to comprise channels with intersecting tunnels. Thealuminum sites are generally acidic, due to the substitution of Al³⁺ inplace of tetrahedral Si⁴⁺ silica requiring the presence of an addedpositive charge. When this positive charge is provided by H⁺, thezeolite is termed HZSM-5.

[0012] HZSM-5 is commercially available from Süd-Chemie Inc.,Louisville, Ky., among other sources.

[0013] In one embodiment, the catalyst can be prepared by a processcomprising (i) binding the HZSM-5 to silica, to form the silica-boundHZSM-5. “Binding” in this context, as used herein, generally refers tophysically admixing HZSM-5 and silica in such a manner that the silicaprovides a support for the HZSM-5. The proportion of HZSM-5 in theadmixture, as a weight percentage of HZSM-5 and silica, can be fromabout 1 wt % to about 90 wt %. In one embodiment, the proportion ofHZSM-5 in the admixture can be from about 2 wt % to about 50 wt %.

[0014] Silica is well known in the art and commercially available from anumber of suppliers.

[0015] The silica-bound HZSM-5 catalyst prepared according to thisembodiment can catalyze the methylation of toluene to xylenes with apara-xylene selectivity in the range of about 65% to 100% of all xylenesproduced (the percentages can be interchangeably expressed by molefraction or by mass).

[0016] In another embodiment, the present invention relates to a methodfor converting toluene to xylenes, comprising:

[0017] (a) contacting toluene with methanol in the presence of asilica-bound HZSM-5 catalyst, wherein the catalyst can be prepared by aprocess comprising: (i) first silylating HZSM-5, to form silylatedHZSM-5; (ii) first calcining the silylated HZSM-5, to form calcinedsilylated HZSM-5; (iii) binding the calcined silylated HZSM-5 to silica,to form silica-bound calcined silylated HZSM-5; (iv) extruding thesilica-bound calcined silylated HZSM-5, to form extruded silica-boundcalcined silylated HZSM-5; (v) second calcining the extrudedsilica-bound calcined silylated HZSM-5, to form extruded silica-boundtwice-calcined silylated HZSM-5; (vi) second silylating the extrudedsilica-bound twice-calcined silylated HZSM-5, to form extrudedsilica-bound twice-calcined twice-silylated HZSM-5; and (vii) thirdcalcining the extruded silica-bound twice-calcined twice-silylatedHZSM-5, to form the silica-bound HZSM-5 catalyst.

[0018] The HZSM-5 is as described above.

[0019] In the first silylating step, the HZSM-5 can be silylated to formsilylated HZSM-5.

[0020] “Silylating,” as used herein, refers to the treating of HZSM-5with silicon compounds selected from the group of alkoxysilanes andpolysiloxanes.

[0021] An alkoxysilane consists of compounds having the formulaSi(R′_(n))(OR)_((4-n)), wherein n is an integer from 0-3, inclusive,each R and R′ is independently a C₁-C₆ alkyl group or a phenyl group.Such alkoxysilanes can be tetramethoxysilane, tetraethoxysilane,tetrapropoxysilane, tetrabutoxysilane, tetraisopropoxysilane,tetraisobutoxysilane and tetrasecbutoxysilane. Exemplary alkoxysilanesinclude, but are not limited to, tetraethoxysilane (TEOS), among others.

[0022] A polysiloxane consists of compounds having the formulaSi(R′_(x))(OR)_((3-x))[Si(R′_(y))(OR)_((2-y))]_(n)OSi(R′_(z))(OR)_((3-z)),wherein x and z are an integer from 0-3, inclusive, y is an integer from0-2, inclusive, n is the degree of polymerization, and each R and R′ isindependently a C₁-C₆ alkyl group or a phenyl group. Polysiloxanesconsist of organosubstituted polysiloxane compounds of varying degreesof polymerization. The only requirement regarding degrees ofpolymerization, for the instant process, is that the viscosity of thefluid be such that the HZSM-5 can be adequately mixed with thepolysiloxane to insure good contact.

[0023] The polysiloxanes can be totally methylated fluids or methyl andphenyl substituted fluids. The polysiloxanes can be either linear orcyclic in structure. The polysiloxanes can be dimethyl and methylphenylsubstituted copolymers.

[0024] The polysiloxanes can be, for example, trimethylsilyl end-blockedmethyl and phenyl substituted polysiloxanes, trimethylsilyl end-blockeddimethylpolysiloxanes, methylphenylhydroxysilyl end-blocked methyl andphenyl substituted polysiloxanes dimethylcyclopolysiloxanes, and methyland phenyl substituted cyclopolysiloxanes. Exemplary polysiloxanesinclude, but are not limited to poly(methyl phenyl)siloxane (PMPS),among others.

[0025] In a silylating step, the HZSM-5 can be contacted with thealkoxysilane according to any appropriate technique, such as a batch orcontinuous process; with the alkoxysilane in the vapor, liquid, or solidphase; among others. If the alkoxysilane is provided in the vapor phase,a carrier or diluent gas, such as nitrogen, helium, argon, carbondioxide, air, or steam, among others, can be used. The alkoxysilaneconcentration in the gas (as vol % relative to the total volume of thegases) can be from about 0.1 vol % to 100 vol %. If the alkoxysilane isprovided in the liquid phase, a solvent, such as benzene, toluene,xylenes, pentane, hexane, heptane, octane, methanol, ethanol, orpropanol, among others, can be used. The alkoxysilane concentration inthe solution (as wt % relative to the total weight of the gases) can befrom about 0.1 wt % to about 10 wt %. Selecting a particular diluent gasor solvent is a matter of routine experimentation for the skilledartisan.

[0026] A silylating step can be performed at any suitable temperatureand for any suitable duration. A suitable temperature for silylatingwith gas phase alkoxysilane can be from about 10° C. to about 600° C.,and with liquid phase alkoxysilane, from about 10° C. to about 150° C.The duration of silylating can be up to about 1 hr, up to about 2 hr, upto about 3 hr, up to about 6 hr, up to about 12 hr, up to about 18 hr,up to about 24 hr, up to about 36 hr, up to about 48 hr, up to about 72hr, or more than about 72 hr. In one embodiment, the duration ofsilylating is at least about 6 hr.

[0027] The amount of the alkoxysilane used to silylate the HZSM-5 canvary. In one embodiment, the amount of the alkoxysilane (on a weightbasis of silicon to the weight of the HZSM-5) to be introduced to theHZSM-5 can be from about 1 wt % to about 10 wt % (i.e., from about 1 gSi as alkoxysilane per 100 g HZSM-5 to about 10 g Si as alkoxysilane per100 g HZSM-5).

[0028] In one embodiment, the first silylating step comprises silylatingthe HZSM-5 with tetraethoxysilane (TEOS).

[0029] The product of the first silylating step is a silylated HZSM-5.

[0030] In the first calcining step, the silylated HZSM-5 can be calcinedto form calcined silylated HZSM-5.

[0031] “Calcining,” as used herein, refers to the treatment of amaterial with heat under an oxidizing environment. The amount of heatapplied can be up to any relatively high temperature, such as about 200°C. or higher, about 300° C. or higher, about 400° C. or higher, about500° C. or higher, or about 600° C. or higher. In one embodiment,calcining is performed at a temperature from about 500° C. to about 600°C. An “oxidizing environment” is any environment, typically a gas, underwhich carbon atoms present in the material can be reacted with compoundsin the environment to form carbon dioxide. Typically, the oxidizingenvironment comprises gaseous oxygen or gaseous air. The duration of thecalcining step is not critical and can be adjusted by the skilledartisan having the benefit of the present disclosure.

[0032] The product of the first calcining step is calcined silylatedHZSM-5.

[0033] In the binding step, the calcined silylated HZSM-5 can be boundto silica, to form silica-bound calcined silylated HZSM-5. The bindingstep can be performed as described above, with the portions of calcinedsilylated HZSM-5 and silica described above.

[0034] In the extruding step, the silica-bound calcined silylated HZSM-5can be extruded to form extruded silica-bound calcined silylated HZSM-5.

[0035] “Extruding,” as used herein, involves the molding of a liquid,powder, or otherwise moldable material into a rigid solid form.Extruding can involve the use of heat or pressure. The term “extruding”encompasses both batchwise molding techniques and continuous moldingtechniques. In one embodiment, the extruding step comprises producing a{fraction (1/16)} inch diameter solid extrudate and pelletizing theextrudate.

[0036] The product of the extruding step is extruded silica-boundcalcined silylated HZSM-5.

[0037] In the second calcining step, the extruded silica-bound calcinedsilylated HZSM-5 is calcined to form extruded silica-boundtwice-calcined silylated HZSM-5. Calcining has been described above.

[0038] In the second silylating step, the extruded silica-boundtwice-calcined silylated HZSM-5 can be silylated to form extrudedsilica-bound twice-calcined twice-silylated HZSM-5. Silylating has beendescribed above. In one embodiment, the second silylating step comprisessilylating the HZSM-5 with poly(methyl phenyl)siloxane (PMPS).

[0039] In the third calcining step, the extruded silica-boundtwice-calcined twice-silylated HZSM-5 can be calcined to form thesilica-bound HZSM-5 catalyst. The silica-bound HZSM-5 catalyst preparedaccording to this embodiment can catalyze the methylation of toluene toxylenes with a para-xylene selectivity in the range of about 65% to100%, such as about 85%, of all xylenes produced.

[0040] In a further embodiment, the present invention relates to amethod for converting toluene to xylenes, comprising (a) contactingtoluene with methanol in the presence of a silica-bound HZSM-5 catalyst,wherein the catalyst can be prepared by a process comprising: (i)binding HZSM-5 to silica, to form silica-bound HZSM-5; (ii) silylatingthe silica-bound HZSM-5, to form silylated silica-bound HZSM-5; (iii)calcining the silylated silica-bound HZSM-5, to form calcined silylatedsilica-bound HZSM-5; (iv) first washing the calcined silylatedsilica-bound HZSM-5, to form washed calcined silylated silica-boundHZSM-5; (v) first drying the washed calcined silylated silica-boundHZSM-5, to form dried calcined silylated silica-bound HZSM-5; (vi)steaming the dried calcined silylated silica-bound HZSM-5, to formsteamed calcined silylated silica-bound HZSM-5; (vii) treating thesteamed calcined silylated silica-bound HZSM-5 with acid, to formacid-treated calcined silylated silica-bound HZSM-5; (viii) secondwashing the acid-treated calcined silylated silica-bound HZSM-5, to formwashed acid-treated calcined silylated silica-bound HZSM-5; and (ix)second drying the washed acid-treated calcined silylated silica-boundHZSM-5, to form the silica-bound HZSM-5 catalyst.

[0041] In the binding step, the HZSM-5 can be bound to silica, to formsilica-bound HZSM-5. The binding of HZSM-5 to silica has been describedabove.

[0042] In the silylating step, the silica-bound HZSM-5 can be silylatedto form silylated silica-bound HZSM-5. Silylating has been describedabove. Any alkoxysilane can be used. In one embodiment, the silylatingstep comprises contacting the silica-bound HZSM-5 with PMPS. The productof the silylating step is a silylated silica-bound HZSM-5.

[0043] In the calcining step, the silylated silica-bound HZSM-5 can becalcined to form calcined silylated silica-bound HZSM-5. Calcining hasbeen described above.

[0044] In the first washing step, the calcined silylated silica-boundHZSM-5 can be washed to form washed calcined silylated silica-boundHZSM-5.

[0045] “Washing,” as used herein, refers to the application of a liquid,such as water, a water-surfactant solution, or an organic solvent, to amaterial. The washing can involve one or more applications of the liquidto the material, and can involve one or more soaking applicationsfollowed by one or more rinsing applications, or one or more combinedsoaking/rinsing applications, among other possibilities that willrecommend themselves to the skilled artisan having the benefit of thepresent disclosure. Washing can be performed at ambient temperature andpressure or adjusted to higher or lower temperature, higher or lowerpressure, or both.

[0046] The product of the first washing step is a washed calcinedsilylated silica-bound HZSM-5.

[0047] In the first drying step, the washed calcined silylatedsilica-bound HZSM-5 can be dried to form dried calcined silylatedsilica-bound HZSM-5.

[0048] “Drying,” as used herein subsequent to washing, refers to theevaporation, either passive or forced, of a liquid used in washing froma material. Forced evaporation can include the use of heat, forced air,or desiccant materials, among other materials or techniques.

[0049] The product of the first drying step is dried calcined silylatedsilica-bound HZSM-5.

[0050] In the steaming step, the dried calcined silylated silica-boundHZSM-5 can be steamed to form steamed calcined silylated silica-boundHZSM-5.

[0051] “Steaming,” as used herein, refers to contacting a material witha gas mixture comprising water vapor and wherein the gas mixture has atemperature of greater than about 100° C., such as from about 500° C. toabout 700° C. The duration of the steaming step is not critical. Typicaldurations of the steaming step can be from about 3 hr to about 12 hr.Generally, though not to be bound by theory, the use of highertemperatures, longer durations, or both in the steaming step will leadto greater dealuminumization in the catalyst. In one embodiment, thesteaming step comprises the use of a gas mixture consisting essentiallyof water vapor at 600° C. for a duration of about 6 hr.

[0052] The product of the steaming step is a steamed calcined silylatedsilica-bound HZSM-5.

[0053] In the treating step, the steamed calcined silylated silica-boundHZSM-5 can be treated with acid, to form acid-treated calcined silylatedsilica-bound HZSM-5.

[0054] “Treating with acid,” as used herein, refers to the contacting ofa material with an acidic solution, i.e., an aqueous solution comprisingan acid and having a pH of less than 7. Any mineral or organic acid canbe used. In one embodiment, the acid can be a mineral acid. In oneembodiment, the mineral acid can be hydrochloric acid (HCl). In oneembodiment, the acidic solution comprises from about 1 wt % HCl tosaturation with HCl (about 38 wt %). In one embodiment, in the treatingstep the acid can be hydrochloric acid at about 0.1 N in aqueoussolution.

[0055] The temperature, duration, and other parameters of the treatingstep are not critical. Generally, the temperature can be from about roomtemperature to about 100° C., and the duration can be from about 1 hr toabout 48 hr, such as from about 6 hr to about 24 hr. In one embodiment,the treating step is performed at about 90° C. for about 16 hr.

[0056] The product of the treating step is an acid-treated calcinedsilylated silica-bound HZSM-5.

[0057] In the second washing step, the acid-treated calcined silylatedsilica-bound HZSM-5 is washed to form washed acid-treated calcinedsilylated silica-bound HZSM-5. The washing step can be performed as isdescribed above.

[0058] In the second drying step, the washed acid-treated calcinedsilylated silica-bound HZSM-5 can be dried to form the silica-boundHZSM-5 catalyst. The drying step can be performed as is described above.

[0059] The silica-bound HZSM-5 catalyst prepared according to thisembodiment can catalyze the methylation of toluene to xylenes with apara-xylene selectivity in the range of about 65% to 100%, such asessentially 100%, of all xylenes produced.

[0060] The following examples are included to illustrate particularembodiments of the invention. However, those of skill in the art should,in light of the present disclosure, appreciate that many changes can bemade in the specific embodiments which are disclosed and still obtain alike or similar result without departing from the spirit and scope ofthe invention.

EXAMPLE 1

[0061] HZSM-5 zeolite was bound to silica (Ludox AS-40) by admixing, andextruded ({fraction (1/16)} in pellets). The extruded material wascalcined (538° C., 6 hr). The silica-bound HZSM-5 (13.4 g) was silylatedwith 26.8 g of a 10 wt % poly(methyl phenyl)siloxane (PMPS) solution incyclohexane. Thereafter, the product was calcined at 538° C. for 6 hr;washed in water; and dried (538° C., 2 hr). 13.7 g of dried calcinedsilylated silica-bound HZSM-5 were produced. An aliquot (6.0 g) of thedried calcined silylated silica-bound HZSM-5 was then subjected to 100%steam, 600° C., for 6 hr. After steaming, the composition was treatedwith 200 g 0.1 N HCl at 90° C. for 16 hr; washed in water; and dried(538° C., 2 hr). A total of 6.0 g of silica-bound HZSM-5 catalyst wasprepared (as a {fraction (1/16)} inch extrudate).

[0062] Thereafter, the catalyst was used to catalyze the methylation oftoluene by methanol. The catalyst was charged to a reactor, and feedscommenced of 20 wt % methanol/80 wt % toluene (20 mL/hr) and hydrogen(260 mL/min). The reactor was maintained at about 600° C. and a pressureof about 70-80 psig. The reaction was allowed to proceed for about 7.5hr, with product quantities measured during the run by GC-FID.

[0063] Table 1 presents the percentage conversion of methanol (i.e., theweight percentage of all methanol added to the reactor minus the weightpercentage of methanol present in the reactor), the weight percentageyield of all xylenes (i.e., the weight of all xylenes present divided bythe weight of all methanol and toluene added to the reactor, multipliedby 100%), and the selectivity of para-xylene (the weight of para-xylenedivided by the weight of all xylenes present, multiplied by 100%) atfour time points. TABLE 1 Methanol Total xylenes yield Para-xylene Time(hr) conversion (wt %) (wt %) selectivity (wt %) 3.25 100.0 11.5 100.04.75 100.0 11.3 100.0 6.00 100.0 10.7 100.0 7.50 100.0 10.7 100.0

[0064] Table 1 indicates that essentially complete selectivity topara-xylene in the methylation of toluene is possible by the use of acatalyst of the present invention.

EXAMPLE 2

[0065] HZSM-5 zeolite was silylated with a 10% solution of tetraethylorthosilicate (TEOS), and then was calcined at 538° C. for 6 hr. Thecalcined silylated HZSM-5 was then bound to silica (Ludox AS-40) byadmixing, and extruded ({fraction (1/16)} inch pellets). The extrudedmaterial was again calcined (538° C. for 6 hr). The calcined extrudedmaterial was silylated with a 10 wt % poly(methyl phenyl)siloxane (PMPS)(Dow 510) solution in cyclohexane and third calcined (538° C. for 6 hr)to produce the silica-bound HZSM-5 catalyst of this example.

[0066] Thereafter, the catalyst was used to catalyze the methylation oftoluene by methanol. The catalyst was charged to a reactor, and feedscommenced of 20 wt % methanol/80 wt % toluene (20 mL/hr) and hydrogen(260 mL/min). The reactor was maintained at about 600° C. and a pressureof about 70-86 psig. The reaction was allowed to proceed for about 7 hr,with product quantities measured during the run by GC-FID.

[0067] At the end of the reaction duration, it was discovered that thereaction exhibited methanol conversion of 99.7 wt % of added methanol;total xylenes yield of 21.5 wt % of added methanol plus added toluene;and para-xylene selectivity of 84.6 wt % of total xylenes.

[0068] This example indicates that methylation of toluene with use of acatalyst of the present invention can produce para-xylene with highselectivity.

[0069] All of the compositions and methods disclosed and claimed hereincan be made and executed without undue experimentation in light of thepresent disclosure. While the compositions and methods of this inventionhave been described in terms of preferred embodiments, it will beapparent to those of skill in the art that variations may be applied tothe compositions and methods and in the steps or in the sequence ofsteps of the method described herein without departing from the concept,spirit and scope of the invention. More specifically, it will beapparent that certain agents which are chemically related may besubstituted for the agents described herein while the same or similarresults would be achieved. All such similar substitutes andmodifications apparent to those skilled in the art are deemed to bewithin the spirit, scope and concept of the invention as defined by theappended claims.

1. A method for preparing a catalyst composition for converting tolueneto xylenes, comprising: (i) first silylating HZSM-5 with analkoxysilane, to form silylated HZSM-5; (ii) first calcining thesilylated HZSM-5, to form calcined silylated HZSM-5; (iii) binding thecalcined silylated HZSM-5 to silica, to form silica-bound calcinedsilylated HZSM-5; (iv) extruding the silica-bound calcined silylatedHZSM-5, to form extruded silica-bound calcined silylated HZSM-5; (v)second calcining the extruded silica-bound calcined silylated HZSM-5, toform extruded silica-bound twice-calcined silylated HZSM-5; (vi) secondsilylating the extruded silica-bound twice-calcined silylated HZSM-5, toform extended silica-bound twice-calcined twice-silylated HZSM-5; and(vii) third calcining the extruded silica-bound twice-calcinedtwice-silylated HZSM-5, to form the silica-bound HZSM-5 catalyst.
 2. Themethod of claim 1, wherein the first silylating step comprisescontacting the HZSM-5 with tetraethoxysilane (TEOS), and the secondsilylating step comprises contacting the extruded silica-boundtwice-calcined silylated HZSM-5 with poly(methyl phenyl)siloxane (PMPS).3. A method for preparing a catalyst for converting toluene to xylenes,comprising: (i) binding HZSM-5 to silica, to form silica-bound HZSM-5;(ii) silylating tie silica-bound HZSM-5, to form silylated silica-boundHZSM-5; (iii) calcining the silylated silica-bound HZSM-5, to formcalcined silylated silica-bound HZSM-5; (iv) first washing the calcinedsilylated silica-bound HZSM-5, to form washed calcined silylatedsilica-bound HZSM-5; (v) first drying the washed calcined silylatedsilica-bound HZSM-5, to form dried calcined silylated silica-boundHZSM-5; (vi) steaming the dried calcined silylated silica-bound HZSM-5,to form steamed calcined silylated silica-bound HZSM-5; (vii) treatingthe steamed calcined silylated silica-bound HZSM-5 with acid, to formacid-treated calcined silylated silica-bound HZSM-5; (viii) secondwashing the acid-treated calcined silylated silica-bound HZSM-5, to formwashed acid-treated calcined silylated silica-bound HZSM-5; and (ix)second drying the washed acid-treated calcined silylated silica-boundHZSM-5, to form the silica-bound HZSM-5 catalyst.
 4. The method of claim3, wherein the silylating step comprises contacting the silica-boundHZSM-5 with PMPS.
 5. The method of claim 3, wherein in the treating stepthe acid is hydrochloric acid at about 0.1 N in aqueous solution.
 6. Themethod of claim 3, wherein the treating step is performed at about roomtemperature to about 100° C. for about 12 hr to about 24 hr.
 7. Themethod of claim 3, wherein the steaming step is performed with about100% water vapor at about 500° C. to about 700° C. for about 3 hr toabout 12 hr.
 8. A catalyst composition for converting toluene toxylenes, prepared by a process comprising: (i) first silylating HZSM-5with an alkoxysilane, to form silylated HZSM-5; (ii) first calcining thesilylated HZSM-5, to form calcined silylated HZSM-5; (iii) binding thecalcined silylated HZSM-5 to silica, to form silica-bound calcinedsilylated HZSM-5; (iv) extruding the silica-bound calcined silylatedHZSM-5, to form extruded silica-bound calcined silylated HZSM-5; (v)second calcining the extruded silica-bound calcined silylated HZSM-5, toform extruded silica-bound twice-calcined silylated HZSM-5; (vi) secondsilylating the extruded silica-bound twice-calcined silylated HZSM-5, toform extruded silica-bound twice-calcined twice-silylated HZSM-5; and(vii) third calcining the extruded silica-bound twice-calcinedtwice-silylated HZSM-5, to form the silica-bound HZSM-5 catalyst.
 9. Thecatalyst composition of claim 8, wherein the first silylating stepcomprises contacting the HZSM-5 with tetraethoxysilane (TEOS), and thesecond silylating step comprises contacting the extruded silica-boundtwice-calcined silylated HZSM-5 with poly(methyl phenyl)siloxane (PMPS).10. A catalyst composition for converting toluene to xylene, prepared bya process comprising: (i) binding HZSM-5 to silica, to form silica-boundHZSM-5; (ii) silylating the silica-bound HZSM-5, to form silylatedsilica-bound HZSM-5; (iii) calcining the silylated silica-bound HZSM-5,to form calcined silylated silica-bound HZSM-5; (iv) first washing thecalcined silylated silica-bound HZSM-5, to form washed calcinedsilylated silica-bound HZSM-5; (v) first drying the washed calcinedsilylated silica-bound HZSM-5, to form dried calcined silylatedsilica-bound HZSM-5; (vi) steaming the dried calcined silylatedsilica-bound HZSM-5, to form steamed calcined silylated silica-boundHZSM-5; (vii) treating the steamed calcined silylated silica-boundHZSM-5 with acid, to form acid-treated calcined silylated silica-boundHZSM-5; (viii) second washing the acid-treated calcined silylatedsilica-bound HZSM-5, to form washed acid-treated calcined silylatedsilica-bound HZSM-5; and (ix) second drying the washed acid-treatedcalcined silylated silica-bound HZSM-5, to form the silica-bound HZSM-5catalyst.
 11. The catalyst composition of claim 10, wherein thesilylating step comprises contacting the silica-bound HZSM-5 with PMPS.12. The catalyst composition of claim 10, wherein in the treating stepthe acid is hydrochloric acid at about 0.1 N in aqueous solution. 13.The catalyst composition of claim 10, wherein the treating step isperformed at about room temperature to about 100° C. for about 12 hr toabout 24 hr.
 14. The catalyst composition of claim 10, wherein thesteaming step is performed with about 100% water vapor at about 500° C.to about 700° C. for about 3 hr to about 12 hr.
 15. A method forconverting toluene to xylenes, comprising: (a) contacting toluene withmethanol in the presence of a silica-bound HZSM-5 catalyst, wherein thecatalyst is prepared by a process comprising: (i) first silylatingHZSM-5, to form silylated HZSM-5; (ii) first calcining the silylatedHZSM-5, to form calcined silylated HZSM-5; (iii) binding the calcinedsilylated HZSM-5 to silica, to form silica-bound calcined silylatedHZSM-5; (iv) extruding the silica-bound calcined silylated HZSM-5, toform extruded silica-bound calcined silylated HZSM-5; (v) secondcalcining the extruded silica-bound calcined silylated HZSM-5, to formextruded silica-bound twice-calcined silylated HZSM-5; (vi) secondsilylating the extruded silica-bound twice-calcined silylated HZSM-5, toform extruded silica-bound twice-calcined twice-silylated HZSM-5; and(vii) third calcining the extruded silica-bound twice-calcinedtwice-silylated HZSM-5, to form the silica-bound HZSM-5 catalyst. 16.The method of claim 15, wherein the first silylating step comprisescontacting the HZSM-5 with tetraethoxysilane (TEOS), and the secondsilylating step comprises contacting the extruded silica-boundtwice-calcined silylated HZSM-5 with poly(methyl phenyl)siloxane (PMPS).17. A method for converting toluene to xylene, comprising: (a)contacting toluene with methanol in the presence of a silica-boundHZSM-5 catalyst, wherein the catalyst is prepared by a processcomprising: (i) binding HZSM-5 to silica, to form silica-bound HZSM-5;(ii) silylating the silica-bound HZSM-5, to form silylated silica-boundHZSM-5; (iii) calcining the silylated silica-bound HZSM-5, to formcalcined silylated silica-bound HZSM-5; (iv) first washing the calcinedsilylated silica-bound HZSM-5, to form washed calcined silylatedsilica-bound HZSM-5; (v) first drying the washed calcined silylatedsilica-bound HZSM-5, to form dried calcined silylated silica-boundHZSM-5; (vi) steaming the dried calcined silylated silica-bound HZSM-5,to form steamed calcined silylated silica-bound HZSM-5; (vii) treatingthe steamed calcined silylated silica-bound HZSM-5 with acid, to formacid-treated calcined silylated silica-bound HZSM-5; (viii) secondwashing the acid-treated calcined silylated silica-bound HZSM-5, to formwashed acid-treated calcined silylated silica-bound HZSM-5; and (ix)second drying the washed acid-treated calcined silylated silica-boundHZSM-5, to form the silica-bound HZSM-5 catalyst.
 18. The method ofclaim 17, wherein the silylating step comprises contacting thesilica-bound HZSM-5 with PMPS.
 19. The method of claim 17, wherein inthe treating step the acid is hydrochloric acid at about 0.1 N inaqueous solution.
 20. The method of claim 17, wherein the treating stepis performed at about room temperature to about 100° C. for about 12 hrto about 24 hr.
 21. The method of claim 17, wherein the steaming step isperformed with about 100% water vapor at about 700° C. for about 3 hr toabout 12 hr.
 22. A xylenes composition, prepared by the method of claim15.
 23. A xylenes composition, prepared by the method of claim 17.